Method of Manufacturing High Annealing Temperature Perpendicular Magnetic Anisotropy Structure for Magnetic Random Access Memory

ABSTRACT

A perpendicular synthetic antiferromagnetic (pSAF) structure and method of making such a structure is disclosed. The pSAF structure comprises a first high perpendicular Magnetic Anisotropy (PMA) multilayer and a second high PMA layer separated by a thin Ruthenium layer. Each PMA layer is comprised of a first cobalt layer and a second cobalt layer separated by a nickel/cobalt multilayer. After each of the first and second PMA layers and the Ruthenium exchange coupling layer are deposited, the resulting structure goes through a high temperature annealing step, which results in each of the first and second PMA layers having a perpendicular magnetic anisotropy.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional application of U.S. application Ser.No. 15/091,853, filed Apr. 6, 2016, which claims the benefit ofProvisional Application No. 62/150,785, filed Apr. 21, 2015. Priority tothis provisional application is expressly claimed, and the disclosure ofthe provisional application is hereby incorporated herein by referencein its entirety.

FIELD

The present patent document relates generally to spin-transfer torquemagnetic random access memory and, more particularly, to a magnetictunnel junction stack having significantly improved performance of thefree layer in the magnetic tunnel junction structure.

BACKGROUND

Magnetoresistive random-access memory (“MRAM”) is a non-volatile memorytechnology that stores data through magnetic storage elements. In a typeof MRAM, the magnetic storage elements comprise two ferromagnetic platesor electrodes that can hold a magnetic field and are separated by anon-magnetic material, such as a non-magnetic metal or insulator. Such astructure is called a magnetic tunnel junction. In general, one of theplates has its magnetization pinned (i.e., a “reference layer”), meaningthat this layer has a higher coercivity than the other layer andrequires a larger magnetic field or spin-polarized current to change theorientation of its magnetization. The second plate is typically referredto as the free layer and its magnetization direction can be changed by asmaller magnetic field or spin-polarized current relative to thereference layer.

MRAM devices store information by changing the orientation of themagnetization of the free layer. In particular, based on whether thefree layer is in a parallel or anti-parallel alignment relative to thereference layer, either a “1” or a “0” can be stored in each MRAM cell.Due to the spin-polarized electron tunneling effect, the electricalresistance of the cell change due to the orientation of the magneticfields of the two layers. The cell's resistance will be different forthe parallel and anti-parallel states and thus the cell's resistance canbe used to distinguish between a “1” and a “0”. One important feature ofMRAM devices is that they are non-volatile memory devices, since theymaintain the information even when the power is off. The two plates canbe sub-micron in lateral size and the magnetization direction can stillbe stable with respect to thermal fluctuations.

Spin transfer torque or spin transfer switching, uses spin-aligned(“polarized”) electrons to change the magnetization orientation of thefree layer in the magnetic tunnel junction. In general, electronspossess a spin, a quantized number of angular momentum intrinsic to theelectron. An electrical current is generally unpolarized, i.e., itconsists of 50% spin up and 50% spin down electrons. Passing a currentthough a magnetic layer polarizes electrons with the spin orientationcorresponding to the magnetization direction of the magnetic layer(i.e., polarizer), thus produces a spin-polarized current. If aspin-polarized current is passed to the magnetic region of a free layerin the magnetic tunnel junction device, the electrons will transfer aportion of their spin-angular momentum to the magnetization layer toproduce a torque on the magnetization of the free layer. Thus, this spintransfer torque can switch the magnetization of the free layer, which,in effect, writes either a “1” or a “0” based on whether the free layeris in the parallel or anti-parallel states relative to the referencelayer.

MRAM devices are considered as the next generation structures for widerange of memory applications. One MRAM technology uses a perpendicularmagnetic tunnel junction device. In perpendicular MTJ devices, the freeand reference layers are separated by a thin insulator layer for spinpolarized tunneling. The free and reference layers have a magneticdirection that is perpendicular to their planes, thus creating aperpendicular magnetic tunnel junction (pMTJ). The pMTJ configurationmay provide a lower critical switching current when compared to in-planeMTJ technology, simplified layer stack structure without need of usingthick antiferromagnetic layers, and reduction of the device size below40 nm.

FIG. 1 illustrates a pMTJ stack 100 for a conventional MRAM device. Asshown, stack 100 includes one or more seed layers 110 provided at thebottom of stack 100 to initiate a desired crystalline growth in theabove-deposited layers. A perpendicular synthetic antiferromagneticlayer (“pSAF layer”) 120 is disposed on top of the seed layers 110. MTJ130 is deposited on top of synthetic antiferromagnetic (SAF) layer 120.MTJ 130 includes reference layer 132, which is a magnetic layer, anon-magnetic tunneling barrier layer (i.e., the insulator) 134, and thefree layer 136, which is also a magnetic layer. It should be understoodthat reference layer 132 is actually part of SAF layer 120, but formsone of the ferromagnetic plates of MTJ 130 when the non-magnetictunneling barrier layer 134 and free layer 136 are formed on referencelayer 132. As shown in FIG. 1, magnetic reference layer 132 has amagnetization direction perpendicular to its plane. As also seen in FIG.1, free layer 136 also has a magnetization direction perpendicular toits plane, but its direction can vary by 180 degrees.

The first magnetic layer 114 in the perpendicular SAF layer 120 isdisposed over seed layer 110. Perpendicular SAF layer 120 also has anantiferromagnetic coupling layer 116 disposed over the first magneticlayer 114. As seen by the arrows in magnetic layers 114 and 132 ofperpendicular SAF 120, layers 114 and 132 have a magnetic direction thatis perpendicular to their respective planes. Furthermore, a nonmagneticspacer 140 is disposed on top of MTJ 130 and a polarizer 150 mayoptionally be disposed on top of the nonmagnetic spacer 140. Polarizer150 is a magnetic layer that has a magnetic direction in its plane, butis perpendicular to the magnetic direction of the reference layer 132and free layer 136. Polarizer 150 is provided to polarize a current ofelectrons (“spin-aligned electrons”) applied to pMTJ structure 100.Further, one or more capping layers 160 can be provided on top ofpolarizer 150 to protect the layers below on MTJ stack 100. Finally, ahard mask 170 is deposited over capping layers 160 and is provided topattern the underlying layers of the MTJ structure 100, using a reactiveion etch (RIE) process.

One of the key challenges in making pMTJ devices is the ability tocreate magnetically stable free and reference layers in the out-of-planeconfiguration for proper device operation. Such reference layers requirehigh perpendicular magnetocrystalline anisotropy (PMA). In order tomagnetically fix the reference layer, a perpendicular syntheticantiferromagnetic structure with high pinning fields (i.e., greater than3 kG) must be formed.

In addition, achieving high perpendicular magnetic anisotropy (PMA) is amajor challenge in development of perpendicular MRAM devices. Suchdevices require a fabrication of the pinned layers with PMA to serve asa reference layer in perpendicularly magnetized MTJs. Moreover, suchstructures need to withstand annealing temperatures up to 400 degreesCelsius for integration with underlying CMOS circuit structures used inconjunction with the MTJs.

Prior approaches to fabricate a pSAF such as SAF 120 used Co/Pd or Co/Ptmultilayers. Perpendicular SAF structures made of Co/Pd or Co/Ptmultilayers, however, fail when annealed at temperatures greater than350 degrees Celsius by losing their perpendicular magnetic orientationand their antiparallel alignment in the pSAF structure. This is one ofthe significant limitations for device CMOS integration, which requiresannealing at temperatures of 350 C or higher.

Thus, there is a need for pSAF for use with an MTJ device, where thepSAF has high PMA and can withstand a high annealing temperature so thatthe MTJ can be integrated with a CMOS device.

SUMMARY

An MRAM device is disclosed that has a magnetic tunnel junction stackand a pSAF that has high PMA and that can withstand high annealingtemperatures. In an embodiment, a magnetic device is described thatcomprises a PMA seed multilayer comprised of a first seed layer and anickel (Ni) seed layer, the Ni seed layer being disposed over the firstseed layer. The device includes a first magnetic perpendicular magneticanisotropy (PMA) multilayer disposed over the PMA seed multilayer, thefirst magnetic PMA multilayer comprising a first cobalt (Co) layer and asecond Co layer, where the first Co layer and the second Co layer areseparated by a first nickel/cobalt (Ni/Co) multilayer. The firstmagnetic PMA multilayer is annealed at a high temperature and has amagnetic direction perpendicular to its plane. The device includes athin Ruthenium (Ru) antiferromagnetic interlayer exchange coupling layerdisposed over the first magnetic PMA multilayer. The device alsoincludes a second magnetic PMA multilayer disposed over the thin Ruantiferromagnetic interlayer exchange coupling layer. The secondmagnetic PMA multilayer comprises a third Co layer and a fourth Colayer, where the third Co layer and the fourth Co layer are separated bya second nickel/cobalt (Ni/Co) multilayer. The second magnetic PMAmultilayer is annealed at the high temperature and has a magneticdirection perpendicular to its plane. The first magnetic PMA multilayer,the thin Ru interlayer exchange coupling layer and the second magneticPMA multilayer form a perpendicular synthetic antiferromagnet.

In an embodiment, the first seed layer comprises alpha phase tantalumnitride.

In an embodiment, the first seed layer has a thickness of fivenanometers.

In an embodiment, the first seed layer comprises tantalum.

In an embodiment, the first seed layer has a thickness of fivenanometers.

In an embodiment, the high temperature is 350 degrees Celsius or higher.

In an embodiment, the first Co layer has a thickness of 0.3 nanometersand the second Co layer has a thickness of 0.18 nanometers.

In an embodiment, the first Ni/Co multilayer comprises a nickel layerhaving a thickness of 0.6 nanometers and a cobalt layer having athickness of 0.2 nanometers.

In an embodiment, the first Ni/Co multilayer is repeated five times.

In an embodiment, the Ni seed layer of the PMA seed multilayer has athickness ranging from 0.5 nanometers to 0.95 nanometers.

In an embodiment, the Ni seed layer of the PMA seed multilayer has athickness of 0.93 nanometers.

In an embodiment, the thin Ruthenium (Ru) antiferromagnetic interlayerexchange coupling layer has a thickness of 0.85 nanometers.

In an embodiment, the first Co layer has a thickness of 0.3 nanometers,the second Co layer has a thickness of 0.18 nanometers, the third Colayer has a thickness of 0.18 nanometers, the fourth Co layer has athickness of 0.18 nanometers.

In an embodiment, the first Ni/Co multilayer comprise a first Ni layerhaving a thickness of 0.6 nanometers and fifth Co layer having athickness of 0.2 nanometers.

In an embodiment, the first Ni/Co multilayer comprises five Ni/Comultilayers.

In an embodiment, the second Ni/Co multilayer comprise a sixth Co layerhaving a thickness of 0.2 nanometers and second Ni layer having athickness of 0.6 nanometers.

In an embodiment, the second Ni/Co multilayer comprises five Ni/Comultilayers.

In an embodiment, the device comprises a non-magnetic tunneling barrierlayer over the second magnetic PMA multilayer and a free magnetic layerover the non-magnetic tunneling barrier layer. The free magnetic layerhaving a magnetic direction that can precess between a first directionand a second direction. The non-magnetic tunneling barrier layerspatially separates the free magnetic layer from the second magnetic PMAmultilayer. The second magnetic PMA multilayer, the non-magnetictunneling barrier layer, and the free magnetic layer form a magnetictunnel junction.

A embodiment for manufacturing a synthetic antiferromagnetic structurehaving a magnetic direction perpendicular to its plane is alsodescribed. The method comprises depositing a PMA seed multilayer. Thestop of depositing of a PMA seed multilayer step comprises depositing afirst seed layer, and depositing a nickel (Ni) seed layer over the firstseed layer.

The method further comprises depositing a first magnetic perpendicularmagnetic anisotropy (PMA) multilayer over the PMA seed multilayer. Thestep of depositing a first magnetic PMA multilayer step comprisesdepositing a first cobalt (Co) layer over the PMA seed multilayer,depositing a first nickel/cobalt (Ni/Co) multilayer over the first Colayer; and depositing a second Co layer over the first Ni/Co multilayer.The second Co layer is separated from the first Co layer by the firstNi/Co multilayer.

The method further comprises depositing a thin Ruthenium (Ru)antiferromagnetic interlayer exchange coupling layer over the firstmagnetic PMA multilayer, and depositing a second magnetic PMA multilayerover the thin Ru antiferromagnetic interlayer exchange coupling layer.The step of depositing of second magnetic PMA multilayer step comprisesdepositing a third Co layer over the thin Ru antiferromagneticinterlayer exchange coupling layer, depositing a second Ni/Co multilayerover the third Co layer, and depositing a fourth Co layer over thesecond Ni/Co multilayer. The fourth Co layer is separated from the thirdCo layer by the second Ni/Co multilayer.

The method further comprises annealing at high temperature for a timesufficient to increase perpendicular magnetic anisotropy of the firstmagnetic PMA multilayer and second magnetic PMA multilayer such that thefirst magnetic PMA multilayer has a magnetic direction perpendicular toits plane and the second magnetic PMA multilayer has a magnetic directperpendicular to its plane.

In an embodiment, the annealing step comprises annealing at 350 degreesCelsius or higher.

In an embodiment, the annealing step further comprises annealing for aperiod of at least two hours.

In an embodiment, the depositing a first magnetic PMA multilayer step isperformed by dc magnetron sputtering.

In an embodiment, the depositing a second magnetic PMA multilayer stepis performed by dc magnetron sputtering.

In an embodiment, the depositing a Ni seed layer over the first seedlayer step comprises depositing the Ni seed layer such that the Ni seedlayer has a thickness of at least 0.93 nanometers.

In an embodiment, the depositing a first seed layer step comprisesdepositing a layer of alpha phase tantalum nitride.

In an embodiment, the depositing a layer of alpha phase tantalum nitridestep comprises depositing a layer of alpha phase tantalum nitride havinga thickness of at least 0.5 nanometers.

In an embodiment, the depositing a thin Ru antiferromagnetic interlayerexchange coupling layer comprises depositing a layer of Ru having athickness of 0.85 nanometers.

In an embodiment, the method further comprises depositing a non-magnetictunneling barrier layer over the second magnetic PMA multilayer; anddepositing a free magnetic layer over the non-magnetic tunneling barrierlayer. The free magnetic layer has a magnetic direction that can precessbetween a first direction and a second direction. The non-magnetictunneling barrier layer spatially separates the free magnetic layer fromthe second magnetic PMA multilayer. The second magnetic PMA multilayer,the non-magnetic tunneling barrier layer, and the free magnetic layerform a magnetic tunnel junction.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included as part of the presentspecification, illustrate the presently preferred embodiments and,together with the general description given above and the detaileddescription given below, serve to explain and teach the principles ofthe MTJ devices described herein.

FIG. 1 illustrates a conventional perpendicular MTJ stack for an MRAMdevice.

FIG. 2 illustrates a process for manufacturing a magnetic device usingthe concepts described herein.

FIG. 3 illustrates the various layers of a magnetic device manufacturedin accordance with the teachings described herein.

FIG. 4 plots the magnetic field (measured in Oe) against the normalizedKerr signal for the magnetic device shown in FIG. 3. Dotted and solidlines represent a device before and after a high temperature annealingstep, respectively.

FIG. 5 illustrates a process for manufacturing a perpendicular syntheticantiferromagnetic structure using the concepts described herein.

FIG. 6 illustrates the various layers of a perpendicular syntheticantiferromagnetic structure manufactured in accordance with theteachings described herein.

FIG. 7 plots the magnetic field in Oe, against the normalized Kerrsignal, which shows the Polar MOKE hysteresis loops of an embodiment asin FIG. 3.

FIG. 8 shows the Polar MOKE hysteresis loops of the structure shown inFIG. 3 having a seed multilayer with β phase TaN layer and a Ni layer.

FIG. 9A shows the Polar MOKE hysteresis loops of a perpendicularsynthetic antiferromagnetic structure according to the present teachingsprior to high temperature annealing.

FIG. 9B shows the Polar MOKE hysteresis loops of a perpendicularsynthetic antiferromagnetic structure according to the present teachingsafter high temperature annealing.

The figures are not necessarily drawn to scale and the elements ofsimilar structures or functions are generally represented by likereference numerals for illustrative purposes throughout the figures. Thefigures are only intended to facilitate the description of the variousembodiments described herein; the figures do not describe every aspectof the teachings disclosed herein and do not limit the scope of theclaims.

DETAILED DESCRIPTION

The following description is presented to enable any person skilled inthe art to create and use a perpendicular synthetic antiferromagneticdevice having high PMA and that is able to withstand high annealingtemperatures so that an MTJ using such a pSAF can be integrated into aCMOS device. Each of the features and teachings disclosed herein can beutilized separately or in conjunction with other features to implementthe disclosed system and method. Representative examples utilizing manyof these additional features and teachings, both separately and incombination, are described in further detail with reference to theattached drawings. This detailed description is merely intended to teacha person of skill in the art further details for practicing preferredaspects of the present teachings and is not intended to limit the scopeof the claims. Therefore, combinations of features disclosed in thefollowing detailed description may not be necessary to practice theteachings in the broadest sense, and are instead taught merely todescribe particularly representative examples of the present teachings.

In the following description, for purposes of explanation only, specificnomenclature is set forth to provide a thorough understanding of thepresent teachings. However, it will be apparent to one skilled in theart that these specific details are not required to practice the presentteachings.

As will now be discussed, the devices described herein have magneticcobalt nickel Co/Ni multilayers deposited directly on specificallydeveloped tantalum (Ta), tantalum nitride (TaN), tantalum nitride/nickel(TaN/Ni) seed layers followed by a high temperature annealing process(i.e., greater than 350 degrees Celsius) to achieve stable perpendicularmagnetized structure. Such structures can be employed when fabricatingperpendicular synthetic antiferromagnet (pSAF) with out-of-plane easymagnetic axis with high coercive field. When employed as pSAF, highpinning fields of up to 5 kG can be achieved. One of the advantages ofthe devices described herein is that stable PMA can be achieved by hightemperature annealing when Ta or TaN, and Ni seed layers are used.

As will be seen, the new structure improves PMA stability for hightemperature annealing and high pinning fields for fabrication ofperpendicular magnetized reference layers for MRAM applications. The newstructure also allows fabrication of a perpendicular polarizing layer inan orthogonal spin transfer torque device.

In perpendicular magnetized structures used in MRAM devices, themagnetization direction of a particular thin film used to manufacture adevice is perpendicular to the plane of the device, and thismagnetization direction exists without application of any externalmagnetic field. Such perpendicular magnetized structures can be achievedby fabricating multilayer structures comprising thin magnetic andnon-magnetic layers such as cobalt (Co), palladium (Pd), nickel (Ni) andplatinum (Pt). Subsequent repetition of such multilayers forces magneticmoments of individual magnetic layers to be directed perpendicular tothe thin film plane. It has been found previously that perpendicularmagnetic anisotropy in these multilayer structures arises from alloyingand forming face-centered cubic (fcc) crystallographic structuresordered with an L1₀ ferromagnetic phase. An fcc crystallographicstructure ordered with an L1₀ ferromagnetic phase has its easy axisalong crystallographic direction, thus forming uniaxial out-of-planeeasy magnetic axis directions.

However, proper seed layers must also be used to obtain uniaxialmagnetic out-of plane symmetry. The seed layer has to provide apreferably fcc ordered template to initiate L1₀ magnetic phase. This isdifficult to achieve because most of the commonly used thermally stablenon-magnetic seed layer, e.g., tantalum (Ta), possess a body-centeredcubic (bcc) crystallographic structure, which prefers in-plane magneticordering. Moreover, seed layer materials such as copper (Cu) orpalladium (Pd) commonly used and are known to have low thermalstability. Copper (Cu) or palladium (Pd) are also known to diffuseduring the annealing process and thus degrade device performance bylowering tunneling magnetoresistance (TMR).

To achieve the benefits of using an fcc crystallographic structurewithout these disadvantages, the various embodiments described hereinutilize cobalt nickel Co/Ni multilayers (which will form the magneticlayers of a synthetic antiferromagnetic structure) that are depositeddirectly by dc magnetron sputtering on tantalum/nickel (Ta/Ni) or alphaphase tantalum nitride/nickel (TaN/Ni) seed layers. As will be discussedin the context of FIGS. 3 and 6, use of an α-TaN/Ni seed layer promotesthe PMA character of Co/Ni structures, but only after annealing. Notethat as used herein, α-TaN refers to alpha phase tantalum nitride.

With reference to FIGS. 2 and 3, an embodiment of a magnetic structure300 in accordance with the present teachings will now be discussed. Aprocess 200 for fabricating a stack of materials forming structure 300having a seed layer that can withstand high annealing temperatures isshown in FIG. 2, while the resulting device 300 is shown in FIG. 3. Atstep 202, a bottom electrode 310 is deposited. Such a bottom electrode310 can be deposited on a semiconductor wafer or other appropriatesubstrate structure. Bottom electrode 310 can comprise a Ta/CuNmultilayer. In one embodiment, the Ta layer of the multilayer has athickness of three nanometers. In other embodiments, the Ta layer canhave a range of 0.5 nm to 10 nm. In an embodiment, the CuN layer of themultilayer has a thickness of forty nanometers. In an embodiment, theCuN layer of the multilayer has a thickness of forty nanometers. Inother embodiments, the CuN layer can have a range of 2 nm to 100 nm.

At step 204, a PMA seed multilayer 360 is deposited. In an embodiment,deposition of PMA seed multilayer is performed in an nitrogen (N2)atmosphere. PMA seed multilayer can comprise several layers. At step206, first seed layer 314 is deposited. First seed layer 314 cancomprise either a Ta layer or an α-TaN layer. At step 208, a Ni layer318 is deposited over first seed layer 314. When first seed layer 314comprises a layer of α-TaN layer, step 206 can be performed bysputtering a phase TaN in an nitrogen (N2) atmosphere. When first seedlayer 314 is a Ta layer, it can have a thickness of five nanometers, andin other embodiments, can have a thickness ranging from 0.5 nm to 10 nm.Likewise, when first seed layer 314 comprises a layer of α-TaN, it canhave a thickness ranging from 0.5 nm to 10 nm. Ni layer 318 can have athickness of 0.93 nm, and in other embodiments, can have a thicknessranging from 0.2 nm to 2 nm.

At step 210, magnetic PMA multilayer 370 is deposited. As seen in FIGS.2 and 3, magnetic PMA multilayer 370 comprises a first cobalt layer 322and a second cobalt layer 330, separated by a Ni/Co multilayer 326. Theprocess of depositing magnetic PMA multilayer 370 (step 210) iscomprised of step 212, in which Co layer 322 is deposited, step 214, inwhich the Ni/Co multilayer 326 is deposited, and step 216, in which Colayer 330 is deposited. Steps 212, 214 and 216 can be performed using dcmagnetron sputtering techniques. Note that magnetic PMA multilayer 370will not actually exhibit any PMA after deposition. In order formagnetic PMA multilayer 370 to exhibit PMA, annealing step, step 222 (tobe discussed) must be performed.

In an embodiment, Co layer 322 has a thickness of 0.3 nm, and in otherembodiments can have a thickness ranging from 0.1 to 0.3 nm. In anembodiment, Ni/Co multilayer 326 can comprise five Ni/Co multilayers,with the Ni layer having a thickness of 0.6 nm and the Co layer having athickness of 0.2 nm. In other embodiments, each Ni/Co multilayer 326 cancomprise Ni layers having a thickness ranging from 0.1 to 0.3 nm and Colayers having a thickness ranging from 0.1 to 3 nm. Likewise, in anembodiment, Co layer 330 has a thickness of 0.18 nm, and in otherembodiments can have a thickness ranging from 0.1 to 0.3 nm.

After depositing the magnetic PMA multilayer 370 in step 210, anyremaining MRAM layers can be deposited, as desired. For example, at step218, remaining MTJ layers (e.g., non-magnetic tunneling barrier layer334 and free layer 338), polarizer layer (not shown), if present, and Rulayer 342 are deposited. Note that this is not required, and therefore,FIG. 2 shows step 218 as an optional step (i.e., with a dotted line).Thereafter, at step 220, cap 390 can be deposited. In an embodiment, cap390 can be made of either a layer of Ta or an α-TaN layer (shown aslayer 346), where the Ta layer (when used) has a thickness of 2 nm whilethe α-TaN layer (when used) has a thickness of 2 nm. In otherembodiments, the Ta layer can have a thickness in the range of 0.5 nm to20 nm while the α-TaN layer can have a thickness in the range of 0.5 to20 nm. Cap 390 can also comprise a Ru layer 350. In an embodiment, Rulayer 350 has a thickness of 5 nm, and in other embodiments, the Rulayer 350 can have a thickness in the range of 0.5 nm to 20 nm.

Depositing Co layers 322 and 330 as well as Ni/Co multilayers 326 on aPMA seed multilayer comprised of either Ta or a α-TaN multilayer, and Nilayer 318 do not necessarily promote the growth of an fcccrystallographic structure ordered with an L1₀ ferromagnetic phase. Thiscan be seen in FIG. 4, which plots the magnetic direction (measure asits field, in Oe) against the normalized Kerr signal. In FIG. 4, thehysteresis loop shown as dotted line 402 demonstrates that as-deposited(i.e, without any annealing), magnetic PMA multilayer 370 (made up ofcobalt and nickel, as discussed), is weakly magnetized in-plane within-plane effective demagnetizing field H_(eff)=1200 Oe.

In the methods and structures described herein, an annealing step 222 isperformed. Annealing at 350 C (or higher) for two hours reverses themagnetic easy axis to the perpendicular direction. This is seen by thesolid line shown in FIG. 4. As is seen, after annealing at 350 degreesCelsius for two hours, polar magneto-optical Kerr Effect (polar MOKE)measurements for structure 300 (without a polarizer or MTJ layers) showperfectly square hysteresis loops measured with a magnetic field appliedperpendicularly to the film plane (along easy magnetic axis). Thus,magnetic PMA multilayer 370 of FIG. 3 has a magnetic directionperpendicular to its plane, which is shown in FIG. 3 by theperpendicular arrow next to magnetic PMA multilayer 370.

With reference to FIGS. 5 and 6, an embodiment of a perpendicularsynthetic antiferromagnetic pSAF structure 600 in accordance with thepresent teachings will now be discussed. A process 500 for fabricating apSAF structure for an MRAM cell 600 having a seed layer that canwithstand high annealing temperatures is shown in FIG. 5, while theresulting device 600 is shown in FIG. 3. At step 502, a bottom electrode510 is deposited. Generally, bottom electrode 510 will be integratedwith CMOS circuitry previously deposited on a semiconductor wafer (notshown). Bottom electrode 510 can comprise a Ta/CuN multilayer. In oneembodiment, the Ta layer of the multilayer has a thickness of threenanometers. In other embodiments, the Ta layer can have a range of 0.5nm to 10 nm. In an embodiment, the CuN layer of the multilayer has athickness of forty nanometers. In other embodiments, the CuN layer canhave a range of 2 nm to 100 nm.

At step 504, a PMA seed multilayer 660 is deposited. In an embodimentdeposition of PMA seed multilayer 660 is performed in a nitrogen N2atmosphere. PMA seed multilayer 660 can comprise several layers. In anembodiment, PMA seed multilayer 660 is formed by depositing, at step506, a first seed layer 614. First seed layer 614 can comprise either alayer of Ta or a layer of α-TaN. At step 508, a Ni layer 618 isdeposited. Step 506 can be performed by depositing a phase TaN bysputtering in N2 atmosphere. In an embodiment, a first seed layer 614 ofTa can have a thickness of five nanometers. Likewise, a first seed layer614 of α-TaN 614 can also have a thickness of five nanometers. In otherembodiments, a first seed layer made of a layer of Ta can have athickness ranging from 0.5 nm to 10 nm. Likewise, in an embodiment, afirst seed layer 614 made of a layer of α-TaN can have a thicknessranging from 0.5 nm to 10 nm. In an embodiment, Ni layer 618 can have athickness of 0.93 nm, and in other embodiments, can have a thicknessranging from 0.2 nm to 2.0 nm.

At step 510, a first magnetic PMA multilayer 670 is deposited. As seenin FIGS. 5 and 6, first magnetic PMA multilayer 670 comprises a firstcobalt layer 622 and a second cobalt layer 630, separated by a Ni/Comultilayer 626. The process of depositing first magnetic PMA multilayer670 (step 510) is comprised of step 512, in which Co layer 622 isdeposited, step 514, in which the Ni/Co multilayer 626 is deposited, andstep 516, in which Co layer 630 is deposited. Steps 512, 514 and 516 canbe performed using dc magnetron sputtering techniques.

In an embodiment, Co layer 622 has a thickness of 0.3 nm, and in otherembodiments can have a thickness ranging from 0.1 to 0.3 nm. In anembodiment, Ni/Co multilayer 626 can comprise five Ni/Co multilayers,with the Ni layer having a thickness of 0.6 nm and the Co layer having athickness of 0.2 nm. In other embodiments, each Ni/Co multilayer 626 cancomprise Ni layers having a thickness ranging from 0.1 to 0.3 nm and Colayers having a thickness ranging from 0.1 to 3 nm. Likewise, in anembodiment, Co layer 630 has a thickness of 0.18 nm, and in otherembodiments can have a thickness ranging from 0.1 to 0.3 nm.

In step 518, an exchange coupling layer 634 is deposited on firstmagnetic PMA multilayer 670. In an embodiment, a thin exchange couplinglayer 634 is comprised of a layer of Ru having a thickness of 0.85 nm.In other embodiments, the Ru layer forming exchange coupling layer 634has thickness ranging from 0.5 nm to 1.5 nm. This thin Ruthenium (Ru)layer 634 is used to provide antiferromagnetic interlayer exchangecoupling between the two components of the pSAF consisting of Co/Nimultilayer structures, i.e., first magnetic PMA multilayer 670,discussed above, and second PMA multilayer 680, discussed below.

At step 520, second PMA multilayer 680 is deposited. In an embodiment,second magnetic PMA multilayer 680 is constructed using the samematerials as first magnetic PMA multilayer 670. However, second magneticPMA multilayer 680 can use different thicknesses for its layers thanfirst magnetic PMA multilayer 670. Likewise, in an embodiment, secondmagnetic PMA multilayer 680 will have a magnetic direction opposite thatof first magnetic PMA multilayer 670, as will be discussed.

As seen in FIGS. 5 and 6, second PMA multilayer 680 comprises a firstcobalt layer 638 and a second cobalt layer 646, separated by a Ni/Comultilayer 642. The process of depositing second PMA multilayer 680(step 520) is comprised of step 522, in which Co layer 630 is deposited,step 524, in which the Ni/Co multilayer 642 is deposited, and step 526,in which Co layer 646 is deposited. Steps 522, 524 and 526 can beperformed using dc magnetron sputtering techniques.

In an embodiment, Co layer 638 has a thickness of 0.18 nm, and in otherembodiments can have a thickness ranging from 0.1 to 0.3 nm. In anembodiment, Ni/Co multilayer 642 can comprise five Co/Ni multilayers,with the Co layer having a thickness of 0.2 nm and the Ni layer having athickness of 0.6 nm. In other embodiments, each Ni/Co multilayer 642 cancomprise Co layers having a thickness ranging from 0.1 to 0.3 nm and Nilayers having a thickness ranging from 0.1 to 3 nm. Likewise, in anembodiment, Co layer 646 has a thickness of 0.21 nm, and in otherembodiments can have a thickness ranging from 0.1 to 0.3 nm. Note thatmagnetic PMA multilayer 370 will not actually exhibit any PMA afterdeposition. In order for first and second magnetic PMA multilayers 670,680 to exhibit PMA, annealing step, step 532 (to be discussed) must beperformed.

At step 528, MRAM layers can be deposited, if desired. For example,layers (e.g., non-magnetic tunneling barrier layer 648 and free layer652) of MTJ 695 and any polarizer layer, if present (not shown in FIG.6), are deposited. Because step 528 is not required, it is shown asoptional (i.e., with a dotted line) to make a functional device in FIG.5. Thereafter, at step 530 cap 690 can be deposited. In an embodiment,cap 690 can be made of either Ta or α-TaN layer 656, where the Ta layer,when used, can have a thickness of 2 nm while the α-TaN layer, whenused, has a thickness of 2 nm. In other embodiments, the Ta layer canhave a thickness in the range of 0.5 nm to 20 nm while the α-TaN layercan have a thickness in the range of 0.5 to 20 nm. Cap 690 also cancomprise a Ru layer 658. In an embodiment, Ru layer 658 has a thicknessof 5 nm, and in other embodiments, Ru layer 658 can have a thickness inthe range of 0.5 nm to 20 nm.

As with the embodiment discussed above with respect to FIGS. 2 and 3,depositing first magnetic PMA multilayer 670 (comprised of Co layers 622and 630 as well as Ni/Co multilayers 626) and second magnetic PMAmultilayer 680 (comprised of Co layers 638 and 646 as well as Co/Nimultilayers 542) on a PMA seed multilayer 660 comprised of a first seedlayer 614 comprising either Ta or α-TaN, and Ni layer 618 do notnecessarily promote the growth of an fcc crystallographic structureordered with an L1₀ ferromagnetic phase. As discussed, this is thereason why the present embodiments perform a high temperature annealingstep 532 once first magnetic PMA multilayer 670 and second magnetic PMAmultilayer 680 have been fabricated. Annealing step 532 can compriseannealing at 350 C for two hours, which as discussed reverses themagnetic easy axis to the perpendicular direction.

Annealing step 532 results in first magnetic PMA multilayer 670 andsecond magnetic PMA multilayer 680 having magnetic directionsperpendicular to their respective planes. This is shown in FIG. 6 by thearrows adjacent to each of first magnetic PMA multilayer 670 and secondmagnetic PMA multilayer 680. As seen in FIG. 6, in an embodiment, themagnetic direction of first magnetic PMA multilayer 670 and secondmagnetic PMA multilayer 680 are perpendicular to their respectiveplanes, but in an antiparallel relationship with one another. Together,first magnetic PMA multilayer 670, second magnetic PMA multilayer 680and thin Ru exchange coupling layer 634 form pSAF structure 685.

In an MRAM device using a structure as in FIG. 6, second magnetic PMAmultilayer 680 of pSAF structure 685 will act as the reference layer ofan MTJ 695 formed with non-magnetic tunneling barrier layer 648 and freelayer 652. Non-magnetic tunneling barrier layer 648 spatially separatesmagnetic free layer 652 from second magnetic PMA layer 680. Firstmagnetic PMA multilayer 670 will act as a pinning layer to the secondmagnetic PMA multilayer 680. In other words, First magnetic PMAmultilayer 670 will act to pin the magnetic direction of the secondmagnetic PMA multilayer 680 so that remains perpendicular to its plane,which allows second magnetic PMA multilayer 680 to act as a referencelayer of an MTJ.

As discussed, one of the benefits of high temperature annealing step 532is that it causes the magnetic direction of first magnetic PMAmultilayer 670 and second magnetic PMA multilayer 680 to becomeperpendicular to each layer's plane. This behavior is influenced inlarge part by Ni layer 618 of PMA seed multilayer 660 and use of lowresistivity a phase TaN in Ta/α-TaN multilayer 614. Increasing thethickness of Ni layer 618 from 0.45 nm to 0.93 nm improves PMA. This isseen in FIG. 7, where increased coercivity H_(c) can be seen fromapproximately 500 to approximately 800 Oe. In particular, FIG. 7 showsPolar MOKE hysteresis loops of an embodiment as in FIG. 3, although theresults for an embodiment as shown in FIG. 6 would be very similar. Thedotted line in FIG. 7 shows the Polar MOKE hysteresis loops for a PMAseed multilayer 360 with a αTaN layer 314 having a thickness of five nmand Ni layer 318 having a thickness of 0.93 nm. The solid line in FIG. 7shows the Polar MOKE hysteresis loops for a PMA seed multilayer having aαTaN layer 314 having a thickness of 5.0 nm and a Ni layer 318 having athickness of 0.45 nm. It can be deducted from FIG. 7 that having either0.45 nm of Ni or 0.93 nm of Ni deposited on 5.0 nm of alpha tantalumnitride induces PMA after annealing. However higher coercivity observedwith 0.93 nm of Ni on 5.0 nm alpha tantalum nitride seed layer indicatesthat PMA reaches a higher value, thus further improving stability of aperpendicular SAF structure.

Moreover, use of high resistive phase of βTaN in multilayer 614 insteadof low resistivity a phase TaN in multilayer 614 in PMA seed multilayer660 shows no PMA. This is seen in FIG. 8. In particular, FIG. 8 showsthe Polar MOKE hysteresis loops of the structure shown in FIG. 3. Thedotted line in FIG. 8 shows the Polar MOKE hysteresis loops where PMAseed multilayer 360 has a βTaN layer 314 and a Ni layer 318 having athickness of 0.93 nm. The dotted line shows the Polar MOKE hysteresisloops prior to annealing. The solid line shows the Polar MOKE hysteresisloops for the structure of FIG. 3 after it has been annealed at 350degrees Celsius for two hours. A person having ordinary skill in the artwill recognize that the round tilted loop indicates that no PMA ispresent. This demonstrates the importance of proper TaN phase forinducing L1₀ ordering during annealing process. In sum, a phase TaNinduces L1₀ ordering during a high temperature annealing process whereas0 phase TaN fails to do so.

The concepts described herein extend to fabrication of the perpendicularsynthetic antiferromagnet structures (pSAF) 685 shown in FIG. 6. Asdiscussed, thin exchange coupling layer 634 made of a film of Ruthenium(Ru) is used to provide antiferromagnetic interlayer exchange couplingbetween the first magnetic PMA multilayer 670 and second magnetic PMAmultilayer 680 (i.e., the two components of pSAF 685). Each of the firstand second magnetic PMA multilayers 670, 680 comprise Co/Ni multilayerstructures, as described above. PMA seed multilayer 660 is comprised ofa seed layer 614 comprised of either Ta or α-TaN, and Ni layer 618, asdescribed above. FIGS. 9A and 9B show Polar MOKE hysteresis loops ofperpendicular synthetic antiferromagnet (pSAF) 685. FIG. 9A shows thePolar MOKE hysteresis loops of pSAF structure 685 as deposited (i.e.,prior to annealing step 532). FIG. 9B shows the Polar MOKE hysteresisloops of pSAF structure 685 after annealing step 532 is performed at 350degrees Celsius for two hours. Square loops, which are seen in FIG. 9B,indicate that pSAF structure 685 after annealing has a magneticdirection that is perpendicular to the plane and thus has aperpendicular easy magnetic axis. As seen in FIG. 9B, strongantiferromagnetic coupling (i.e., a high pinning field) is present afterannealing. The high pinning field, H_(ex), equals 5 kOe.

Thus, as is seen, use of a PMA seed multilayer 660 comprised of a Talayer 614 and Ni layer 618 or an α phase TaN layer 614 and a Ni layer618 for growth of first magnetic PMA multilayer 670 and second magneticPMA multilayer 680 allows the structure to withstand a high annealingtemperature, e.g., an annealing temperature of 350 degrees Celsius orhigher. Previous solutions could not achieve high pinning fields withPMA after annealing above 250 C.

In another embodiment, one can use pSAF structure 685 as describedherein as a polarizer layer in an orthogonal spin transfer torquestructure.

It should be appreciated to one skilled in the art that a plurality ofstructures 600 can be manufactured and provided as respective bit cellsof an MRAM device. In other words, each structure 600 can be implementedas a bit cell for a memory array having a plurality of bit cells. Theabove description and drawings are only to be considered illustrative ofspecific embodiments, which achieve the features and advantagesdescribed herein. Modifications and substitutions to specific processconditions can be made. Accordingly, the embodiments in this patentdocument are not considered as being limited by the foregoingdescription and drawings.

1. A method of manufacturing a synthetic antiferromagnetic structurehaving a magnetic direction perpendicular to its plane, comprising:depositing a PMA seed multilayer, wherein the depositing of a PMA seedmultilayer step comprises: depositing a first seed layer, and depositinga nickel (Ni) seed layer over the first seed layer; depositing a firstmagnetic perpendicular magnetic anisotropy (PMA) multilayer over the PMAseed multilayer, wherein the depositing of a first magnetic PMAmultilayer step comprises: depositing a first cobalt (Co) layer over thePMA seed multilayer; depositing a first nickel/cobalt (Ni/Co) multilayerover the first Co layer; and depositing a second Co layer over the firstNi/Co multilayer, wherein the second Co layer is separated from thefirst Co layer by the first Ni/Co multilayer; depositing a Ruthenium(Ru) antiferromagnetic interlayer exchange coupling layer over the firstmagnetic PMA multilayer; depositing a second magnetic PMA multilayerover the thin Ru antiferromagnetic interlayer exchange coupling layer,wherein the depositing of second magnetic PMA multilayer step comprises:depositing a third Co layer over the thin Ru antiferromagneticinterlayer exchange coupling layer; depositing a second Ni/Co multilayerover the third Co layer; and depositing a fourth Co layer over thesecond Ni/Co multilayer, wherein the fourth Co layer is separated fromthe third Co layer by the second Ni/Co multilayer; annealing at atemperature of 350 degrees Celsius or higher for a time sufficient toincrease perpendicular magnetic anisotropy of the first magnetic PMAmultilayer and second magnetic PMA multilayer such that the firstmagnetic PMA multilayer has a magnetic direction perpendicular to itsplane and the second magnetic PMA multilayer has a magnetic directperpendicular to its plane.
 2. (canceled)
 3. The method of claim 2,wherein the annealing step further comprises annealing for a period ofat least two hours.
 4. The method of claim 1, wherein the depositing afirst magnetic PMA multilayer step is performed by dc magnetronsputtering.
 5. The method of claim 1, wherein the depositing a secondmagnetic PMA multilayer step is performed by dc magnetron sputtering. 6.The method of claim 1, wherein the depositing a Ni seed layer over thefirst seed layer step comprises depositing the Ni seed layer such thatthe Ni seed layer has a thickness of at least 0.93 nanometers.
 7. Themethod of claim 1, wherein the depositing a first seed layer stepcomprises depositing a layer of alpha phase tantalum nitride.
 8. Themethod of claim 7, wherein the depositing a layer of alpha phasetantalum nitride step comprises depositing a layer of alpha phasetantalum nitride having a thickness of at least 0.5 nanometers.
 9. Themethod of claim 1, wherein the depositing a thin Ru antiferromagneticinterlayer exchange coupling layer comprises depositing a layer of Ruhaving a thickness of 0.85 nanometers.
 10. The method of claim 1,further comprising: depositing a non-magnetic tunneling barrier layerover the second magnetic PMA multilayer; and depositing a free magneticlayer over the non-magnetic tunneling barrier layer, the free magneticlayer having a magnetic direction that can precess between a firstdirection and a second direction, the non-magnetic tunneling barrierlayer spatially separating the free magnetic layer from the secondmagnetic PMA multilayer, wherein the second magnetic PMA multilayer, thenon-magnetic tunneling barrier layer, and the free magnetic layer form amagnetic tunnel junction.